Hands down, Knoebels! As nearly everyone else has told you, Phoenix and Twister are fantastic!
However, if you like steel, you won't want to miss Dorney. As a park, it's not one of my personal favorites, but Steel Force is great and the Lazer is the only consecutive double-looper I've been on. The woodies and other rides disappoint me.
Hershey probably has the nicest variety of the PA parks. Great Bear and Wildcat are excellent, and I can hardly wait to ride the new Lightning Racer. SooperDooperLooper and Comet are also really nice rides.
And Kennywood is like a museum! I mean this in a positive sense. They have three very unique woodies. They are all pretty modest in size but I've had a great time on all 3! Plus, this is Steel Phantom's last year. Probably many enthusiasts will be out to ride it this summer. It's been highly criticized, but it's one of my favorite coasters. The second drop is one of the best I've ever experienced.
Have fun this summer! PA is great for coastering!